The core in a nuclear reactor uses a large number of precision manufactured nuclear fuel rods containing fissionable pellets therein. The fuel rods typically are manufactured from open-ended tubular rods made from a zirconium alloy. One end of the rod is plugged with a zirconium alloy end plug and girth welded by conventional tungsten inert gas methods. The fissionable pellets are inserted into the rod through its open end which then is plugged and girth welded. Afterward, the rod is pressurized and seal welded.
Weld quality is a paramount concern in the manufacture of these rods. During fission, gas pressure builds in a fuel rod. A poor weld could create a gas leak at the weld seam resulting in rod depressurization and increased coolant radioactivity. If the rod depressurizes, the high pressure attendant the reactor core could collapse the rod creating reactor operational problems. Thus, before the rods are inserted into a reactor core, their welded ends are inspected to assure a strong weld has been obtained.
Most nuclear fuel rods are inspected in apparatus using X-ray analysis. Typically, nuclear fuel rods are serially guided through a housing having a pair of longitudinally separated slots for receiving X-rays downwardly from an external source so as to define first and second longitudinally spaced apart operating positions. The rods are guided through the housing to a first rod position where the forward ends of the rods are aligned below the first operating position and to a second rod position where the rear ends of the rods are aligned below the second operating position. X-ray film cartridges are serially advanced through the housing below the rods on an endless conveyor belt and exposed to X-rays to image each end of the fuel rod. In prior art systems, the conveyor is operated unidirectionally and includes spaced-apart cleats for defining receptacles to hold the film cartridges therein.
Usually, the conveyor is advanced incrementally so that multiple exposures of each rod end are taken on one film cartridge. The rods are rotated a fixed angular amount before each exposure to obtain an image of the fuel rod end in a different orientation. Additionally, each film cartridge is exposed to image both front and rear ends of a rod by guiding a rod from the first operating position to the second operating position and advancing the conveyor to move a film cartridge from the first to the second operating position. Multiple exposures are taken again.
As the conveyor is advanced, one completed film cartridge is removed and a new cartridge placed onto the conveyor. The new film cartridge is positioned on the conveyor and aligned below the first operating position so that simultaneous with the multiple exposure of the rear end of a first rod which has been advanced to the second operating position, the front end of a new rod which has been guided into the housing is exposed at the first operating position. The cycle continues as rods are guided through the housing and the conveyor is advanced so that exposed film cartridges are removed from one end of the conveyor near the second operating position and new film cartridges are positioned on the conveyor near the first operating position.
One drawback of the prior art practice is the error accumulation attendant any mechanical conveyor system. Each multiple exposure of a rod end requires a predetermined incremental advancing of the film cartridge below the rod end a distance at least as great as the width of the slot through which X-rays pass to prevent overlap of successive exposures on the film cartridge. However, because the conveyor is unidirectional, any mechanical error is amplified. This accumulated error makes accurate incremental positioning of a film cartridge difficult causing overlap between exposures resulting in poor quality X-ray images.
It is therefore an object of the present invention to provide an apparatus and method for inspecting the quality of both ends of nuclear fuel rods which overcomes the above-noted deficiencies of the prior art practices.
It is a more particular object of the present invention to provide an apparatus and method for inspecting the quality of both ends of nuclear fuel rods wherein the mechanical error normally associated with a conveyor system used in advancing film cartridges serially through an X-ray housing of the above-noted prior art practice is minimized.
It is still another object of the present invention to provide an apparatus and method for inspecting the quality of both ends of nuclear fuel rods wherein a film cartridge positioned at a first receptacle can be transferred to a second receptacle to eliminate the use of a unilateral conveyor system so as to minimize mechanical error.